The present invention relates generally to optical spectroscopy systems and methods and more specifically to systems and methods for controlling the optical path length between a laser and cavity in optical spectroscopy systems.
In cavity enhanced optical absorption spectroscopy systems and methods, radiation of a laser is directed into a resonance cavity, and the optical intensity inside the cavity is observed. The optical frequency of the laser can be periodically scanned. If it is assumed for clarity that the laser linewidth is much smaller than the cavity resonance width, at the moment when the laser light frequency coincides with a cavity mode transmission peak the optical intensity inside the resonance cavity reflects total cavity loss, and the total cavity loss can be quantitatively determined provided that the incident intensity and cavity parameters are known. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of a gas mixture present in the cavity. The lower the cavity mirror losses, or equivalently, the higher each mirror's reflectivity—the smaller the absorption of the intra-cavity gas mixture that can be detected. With very high reflectivity mirrors, the laser linewidth will become too large compared to the cavity resonance width, thus limiting achievable enhancement of the gas mixture absorption by the cavity. This can be helped by narrowing the laser linewidth using optical feedback from the cavity and a laser that is sensitive or responsive to optical feedback from the cavity. With such a laser during the scan, as the frequency of the laser light approaches the frequency of one of the cavity modes, the laser locks to that mode. By saying that the laser is locked to the mode it is meant that the laser linewidth becomes much smaller than the resonance mode width, and that regardless whether the frequency scan range of the unlocked laser may be large, in a locked condition the optical frequency of the laser will change only within the resonance peak. As the laser frequency scan continues, the laser will lose the lock to the current cavity mode and relock to the next cavity mode that it approaches. Due to the optical feedback effect, the laser optical frequency during the scan will essentially take a number of discrete values corresponding to the peaks of the cavity mode resonances that are equidistant in optical frequency. A discrete absorption spectrum of the analyzed gas can thus be obtained by sequential coupling to the entire set of the cavity modes within the scan range, and the trace gas concentration can be derived from the absorption spectrum. This sub-family of cavity enhanced optical absorption spectroscopy systems and methods that uses optical feedback will be referred to as optical feedback cavity enhanced absorption spectroscopy (OF CEAS). For simplicity, optical feedback assisted cavity ring-down spectroscopy is included in the OF CEAS definition.
In OF CEAS, the strength of the optical feedback from the resonance cavity to the laser has to be within certain limits, otherwise it is impossible to provide reproducible scan-to-scan mode coupling as the laser scans. In OF CEAS systems and methods that are known so far, complex optical components are used for this purpose, such as Faraday isolators, variable optical attenuators, or polarization rotators. Adverse interference effects, temperature drifts and aging drifts may result from these components in the system. Achieving high stability and high reproducibility of the optical absorption measurements becomes a major problem.
In OF CEAS, one traditional configuration includes a linear V-cavity, a DFB laser coupled to the cavity though a vertex mirror, and one or more photo-detectors positioned to monitor optical powers, e.g., the intra-cavity optical power circulating within the cavity. The intra-cavity optical power buildup, while the laser is scanned over cavity modes, or free decay of the intra-cavity optical power after the laser is turned off, is defined by the cavity loss. When optical feedback is used to lock a laser to a cavity, its phase must be controlled with high precision. To provide this control, the laser-cavity optical distance must be controlled with sub-micron accuracy. Traditionally, this control is performed by placing one of the beam delivery mirrors on a piezo transducer (PZT) to control the optical path length between the laser and the cavity. Alternatively, the laser itself can be mounted on a piezo transducer. However, it is known that piezo transducers produce wobbling motion while they are expanding or contracting. This effect might cause a partial misalignment of the laser relative the cavity. Piezo transducers also tend to produce a hysteresis effect. For example, when it is desirable to change the optical path length by a specific amount (e.g., half a wavelength or a full wavelength), it can be difficult to determine how much voltage to apply relative to the previous value due to hysteresis. Moreover, the applications which use high voltage power suppliers to drive piezo transducers are restricted.
Therefore it is desirable to provide OF CEAS systems and methods that overcome the above and other problems, and in particular the problems associated with the use of piezo transducers or similar elements.